*** The Ampliphase Page ***

Ampliphase - A quick description....

The theory of Ampliphase was proposed and developed by Henry Chireix in 1935 who named the technique "outphasing". The marketing name of Ampliphase was adopted by RCA when they launched the BTA-50G transmitter using this technology in 1956. In 1960 the BTA-50H was launched, and although similar to the 50-G, it features solid state silicon rectifiers throughout, new "lightweight" tubes, and an improved modulator system.
RCA also used the same technology to produce five (BTA-5L), ten (BTA-10J/L) and one-hundred (BTA-100B) kilowatt transmitters for AM broadcast. In addition, a limited number of short wave ampliphase transmitters were also produced (BHF100A) though the design of this was significantly different than the standard broadcast units. A number of European manufacturers also built Ampliphase type systems, notably Marconi in the UK and Thomson in France.
For a discussion of the development of various ampliphase transmitters see the ancestry page in the links at the bottom of this page.

The principle of Ampliphase is to take a single frequency source, split it into two feeds, phase modulate each feed, then add the two signals in a combining network. Amplitude modulation is achieved by the degree of
addition or subtraction due to the phase difference between the the two signals. All (?) Ampliphase transmitters are arranged so that under zero modulation conditions, the quiescent phase difference is 135 degrees. The two signals are modulated in equal but opposite directions by a total of +/- 45 degrees, giving maximum and minimum differences of 180 and 90 degrees.
At 180 degrees, the two waves cancel out in the combining network, hence this is the 100% peak negative modulation condition. At 90 degrees, the two waves add to give close to 100% positive modulation.

Ampliphase transmitters quickly acquired the nickname of "Amplifuzz" due to the quirky nature of setting them up and maintaining consistant performance, and became the transmitters engineers loved to hate. Although the technique became obsolete for broadcast use in the 1970's, outphasing has undergone a bit of a renaissance in recent years in the microwave spectrum. It is used as an efficient and accurate means of producing phase/amplitude (QUAM) modulated wideband carriers, necessitated by todays digital communications systems.

The notes on these pages refer to the BTA-50H, although the same modulator chassis was used in the 50G/50H1 and 100B AM broadcast transmitters. If you can add any information (or spot any mistakes) to these pages, please feel free to contact me at the address at the bottom of the page. I am, in particular, keen to hear from you if you are going to be disposing of an Ampliphase transmitter, or have parts available.

Fig 1: The Block diagram of the 50H from the handbook. (click to enlarge)

From the block diagram (above), it can be seen that the frequency source is a crystal oscillator (at the operating frequency) based on an 807. This oscillator module was common to many other RCA BTA series transmitters from the 1940's through to the 1960's. The first part of the ampliphase process is applied by a 5693 pentode which drives a transformer load,
producing bi-phase outputs, 180 degrees apart. These outputs are then fed to two identical modulation chains, each comprising of four cascaded stages comprising of a 5693 "modulation shifter" amplifier and 5692 "phase modulator" triode. The first stage is "DC modulated" by a variable resistance (no triode in this stage) which is used to set the quiescent phase difference at 135 degrees,
hence the transmitters nominal output power. The three subsequent stages have audio fed to the grids of the triodes. The anode load of each pentode amplifier is effectively a tuned circuit comprising of a
parallel L/C network with a series component formed by the anode/cathode conductance of the triode. As audio is applied to the triode grid, its conductance thus varies. This has the effect of being a variable resistance in the L/C resonator, hence producing instantaneous phase change which corresponds to the applied audio.
Cascaded stages are used to ensure linearity of modulation, as a single stage could not provide enough phase change in a linear mode. Equal but opposite modulation is achieved by feeding the two chains with anti-phase audio derived from dual windings on the audio input isolating transformer. As the total phase change required is +/-45 degrees
one chain must reach +45 degrees whilst the other reaches -45 degrees. Each stage of active modulation therefore produces just 15 degrees of modulation.

Above : The modulator layout from the handbook.

Below : The modulator sitting on my dining room table!

Note the symmetrical layout.

Opened for inspection

The two outputs of the modulation chains are amplified by separate conventional cascaded class C RF stages, (4-250 tetrodes at 1KV anode and 4CX5000 tetrodes at 5KV anode), culminating in PA stages based on "lightweight" 6697 triodes operating at 15KV anode.
These 6697's are tuned with a conventional tank circuit, and it is the output of these two tank circuits which are combined to generate the modulated carrier. PA tuning is complicated due to two phase shifted signals coming together. One PA will see the leading phase of the other PA in the combining point as a capcitive load, whilst the second PA will see the lagging load as being inductive. The tank circuit must also be arranged such that it presents a 90 degree (1/4 wave) phase shift between the PA plate and the combining point.
After the combining point conventional T and Pi filters and harmonic traps are utilised to match into the load.

Above: 6697 PA tube alongside fire extinguisher for scale

One drawback to the process just described is that although the audio to phase modulation process is linear, the process of combining two waveforms is not. It is actually a co-sinusoidal function of the phase difference. Think of it this way:- If we equate 0 - 100% input to be 0 - 90 degrees phase change, then at 50% input,
we have a phase difference of 45 degrees. However, the amplitude of a sinewave at 45 degrees is in fact 0.707 . Thus with 50% input we have 70.7 % output! And accordingly at 33.3% (30 degrees) input we have 50% output and at 66.6% (60 degrees) input, 86.6% output. Hence we have a linearity problem causing compression of positive modulation.
In reality, the phase addition is a function of half the phase angle, so the input level for 100% modulation will increase the carrier to 185% of the nominal level. To counteract this dependancy and improve the modulation efficiency, audio modulation is processed by the "linearity corrector" and superimposed on the bias of the driver stages to the PA's.
Under high positive modulation, the bias voltage is reduced, allowing greater anode conduction and higher power. Under neagtive trough modulation, the driver bias is increased to reduce the drive to the PA stages - this prevents the two stages producing 25KW of energy only to be cancelled out in the combiner. Reducing the drive level has the added benefit of
compensating for inequalities of the power output of the two stages as well as reducing overall power consumption. These features are combined with a negative feedback network (to enhance the transmitter stabilty and distortion) to create the "drive regulator chassis".

Above: Trapezoid modulation curve without linearity correction

An alternative view of understanding the PA combining and drive regulator process, is to think about the output as being "load impedance modulation". The plate tank circuits will normally give a phase change of 90 degrees (1/4 wave) between input and output. At the point of 100% negative modulation, the two opposing waves will cancel out in the common load, looking like a short circuit. This will be reflected through the 1/4 wave tank into an open circuit in the plate circuit
(thus no power is actually supplied to the load). As the two waves come closer together in phase, the perceived impedance of the load will increase, which again will be reflected through the tank circuit to the anodes. A contantly varying load is therefore seen by the PA anodes, and the drive regulator must "adjust" the bias under modulation conditions to keep the plate voltage/current characteristics suited to the apparent impedance and amount of power required by the load.

Above : The drive regulator.

In 1969 RCA brought out the BTA-50J which had a newly developed solid state exciter/modulator. This unit could also be retrofitted to the 50H, to make it a 50HS. (S for solid state) It was a very complex piece of kit, and there are mixed reports of its performance. Some idea of its complexity can be gauged from
the fact the handbook for this modulator is actually bigger than the handbook for the entire transmitter! Its principal of operation was also somewhat different from the cascaded phases modulators of the original system. Following buffering, the crystal frequency was converted into a symmetrical triangular wave, and fed through a "slicer" comparator circuit. Also fed into the comparator was the audio. The output from the slicer was a pulse width modualted waveform, on the carrier frequency.
A narrow pulse width corresponded with high levels of positive audio, and a wide pulse width with negative levels of audio. The rising and falling edges of the PWM wave were separated and differentiated to produce two "spikes". For positive audio (narrow pulse) the spikes would be close together, for low level audio, the spikes would be further apart. These two spikes were then used to trigger separate monostable circuits, each monostable timed to have a duty cycle of 120 degrees at the carrier frequency.
The outputs of the monostables were used to drive the two Class-C amplifier chains of the transmitter as described above. This exciter arrangement was essentially broadband, though the triangle waveform had to be set to be symmetrical on the carrier frequncy, and the two monostables needed to be adjusted to be 120 degree duty cycle. A separate solid state "baseband" amplifer module within this exciter provided for the drive regulation control to the grids of the driver tubes.
The same solid state exciter was also fitted as standard to the BTA5 and 10 L series of ampliphase transmitters.

The pro's and con's of ampliphase are varied. It can achieve excellent modulation fidelity and high transmitter efficiency compared to a plate modulated transmitter (no need for 25KW audio amplifiers and modulation transformer!) but does require two identical RF PA stages and a combining network, and can be a problem to set up and maintain identical performance
for the two modulation chains (any mismatch in the phase shift or level between the two chains results in excesive phase shift of the final carrier). It was also necessary to maintain all the tuned stages within the transmitters as broad as possible to prevent any inadvertant phase distortion which would result in sideband splatter on the final output.

Today, of course, the whole modulation stages could be built into one chip of silicon, complete with RF frequency synthesizer,
audio processing, logic-control and probably most of the studio playout system(!), although digital modulation systems, resulting in 90% or better TX efficiency have rendered the technique obsolete for broadcast use.

I hope the above description is accurate - I have written it from studying the BTA50H handbook and various materials produced by RCA some 40 odd years ago. Please feel free to contact me with any comments, corrections, or other info. As stated earlier, I would love to make contact with engineers who worked on this rigs in their hey-day, or anyone who may have parts or other information to share. - A big thank you to John, Barry, Don, Duffy, Grady and others who have already assisted me.

This page will be updated as and when I have extra information to add. Other pages on this website are updated more frequently, please use the menu bar on the left hand side.

Wanted! Wanted! Wanted!

I am looking for used 6697 or 8773 tubes of the glass bulb variety (ie 6697, not the ceramic 6697A). As this transmitter restoration project
is solely for display purposes general condition and emission is not important. However the filament and vacuum must be intact so it can be viewed by the
public. Please contact me via the feedback form on the left menu !

Also, I would not say no to any good condition 4CX5000's you may wish to dispose of.

My E-mail address is : "ampliphase [at] radio [dot] fm" or use the links to the feedback form. Thanks!